Ancient calendars were remarkably accurate but over the centuries it is hard to compare the dates of one to the modern day calendar. Things change after all and no calendar is perfect in an ever changing world. The Maya are famous for their complex, intertwined calendar systems, and now one calendar, the Maya Long Count, has been empirically calibrated to the modern European calendar, according to an international team of researchers. Archaeologists want to place the Long Count dates into the European calendar so there is an understanding of when things happened in the Maya world relative to historic events elsewhere. Correlation also allows the rich historical record of the Maya to be compared with other sources of environmental, climate and archaeological data calibrated using the European calendar.

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The Mesoamerican Long Count calendar is a non-repeating, vigesimal (base-20) and base-18 calendar used by several Pre-Columbian Mesoamerican cultures, most notably the Maya. For this reason, it is sometimes known as the Maya (or Mayan) Long Count calendar. Using a modified vigesimal tally, the Long Count calendar identifies a day by counting the number of days passed since a mythical creation date that corresponds to a presently estimated date of August 11, 3114 BCE.

Over the history of ancient Maya studies, a wide variety of Christian calendar correlation dates have been suggested. Today, the majority of scholars fall into one of two camps - the GMT and the GMT+2. GMT stands for Goodman-Martinez-Thompson, the three early scholars who compiled the evidence correlating the Julian calendar (in use during the initial European contact) with the Maya calendar. The GMT+2 has gained popularity because certain carved monuments with astronomical data, such as the solar eclipse recorded on a stela from Poco Uinic in Chiapas, correlate better with two days after the original GMT.

"The Long Count calendar fell into disuse before European contact in the Maya area," said Douglas J. Kennett, professor of environmental archaeology, Penn State. "Methods of tying the Long Count to the modern European calendar used known historical and astronomical events, but when looking at how climate affects the rise and fall of the Maya, I began to question how accurately the two calendars correlated using those methods."

The researchers found that the new measurements mirrored the most popular method in use, the Goodman-Martinez-Thompson (GMT) correlation, initially put forth by Joseph Goodman in 1905 and subsequently modified by others. In the 1950s scientists tested this correlation using early radiocarbon dating, but the large error range left open the validity of GMT.

"With only a few dissenting voices, the GMT correlation is widely accepted and used, but it must remain provisional without some form of independent corroboration," the researchers report in the April issue of Scientific Reports.

A combination of high-resolution accelerator mass spectrometry carbon-14 dates and a calibration using tree growth rates showed that the GMT correlation is correct.

The Long Count counts days from a mythological starting point. The date is comprised of five components that combine a multiplier times 144,000 days — Bak'tun, 7,200 days — K'atun, 360 days — Tun, 20 days — Winal, and 1 day — K'in separated, in standard notation, by dots.

Archaeologists want to place the Long Count dates into the European calendar so there is an understanding of when things happened in the Maya world relative to other historic events elsewhere. Correlation also allows the rich historical record of the Maya to be compared with other sources of environmental, climate and archaeological data calibrated using the European calendar.